The present invention relates to reinforced paperboard cartons and, more particularly, it relates to selective reinforcement of paperboard cartons by area-wise impregnation of strategic portions of the cartons with a polyisocyanate.
Paperboard cartons are widely used in the packaging area. For example, the Mead Corporation, assignee of the present invention, markets under the trademark Cluster-Pak, a paperboard carton for carrying glass bottles such as bottles of soft drinks. That carton is manufactured under one or more of the following U.S. Pat. Nos.: 3,169,691; 3,233,483; 3,517,876; 3,519,127; 3,807,624; 4,077,095; 4,084,693; and 4,093,116.
Generally, the Cluster-Pak carton has at the top a pair of fingerholes and a series of circular openings through which the glass soft drink bottle necks extend. These fingerholes and circular openings are die cut from the paperboard either before or after coating and printing. Another series of apertures is die cut in the base of the carton so that the paperboard blank can be appropriately folded and mechanically locked to form the final package.
As a result of the formation of fold lines and die cut holes, openings and apertures, areas of weakness are formed in the paperboard carton. In particular, it has been found that two locations are prone to tensile failure: (1) the fingerholes in the top of the carton, and (2) the apertures or "heels" in the bottom of the carton which are adjacent the base of the glass soft drink bottle. The fold lines are also areas of concern.
In any of these areas if there is a significant failure, the whole carton or one or more bottles may drop from the carrying hand, hit the floor, and break. Obviously, such losses are to be avoided if at all possible. As a result, it has been the practice to use a heavier basis weight paperboard than might otherwise be required in order to provide a margin of safety in terms of potential tensile failures. This is expensive and, even then, failures do on occasion occur.
Accordingly, it would be desirable to reinforce the paperboard carton in order to prevent such failures. However, previous efforts to do so have a number of drawbacks. An overall plastic coating or lamination is expensive. Adhering bands of selective reinforcement at strategic locations requires special equipment and results in a non-uniform increase in thickness of the paperboard blank which presents stacking, shipment and handling problems to the soft drink bottlers.
Of course, it is known that impregnation with certain resins may increase compressive strength, tensile strength, tearing strength, flexibility, moisture penetrability, etc., of paperboards without neccessarily increasing the thickness of the paperboard blank. See, for example, Igarashi et al, U.S. Pat. No. 3,953,169 wherein it is disclosed that a paperboard impregnated with polyurethane resin having a specific NCO content and an average molecular weight at the rate of 0.05 to 5.0 weight percent relative to the paperboard has excellent mechanical properties such as compressive strength, tensile strength, tearing strength, folding endurance, etc. even in a wet state. The polyurethane resin of Igarashi et al may be either a one-component polyisocyanate derived from tolylene diisocyanate or a two-component polyisocyanate composition of polyol and polyisocyanate derived from tolylene diisocyanate. It is applied from an organic solvent solution by dipping or coating and then dried by steam heating at approximately 130.degree. C. for about 5 seconds. See also LaCombe et al, U.S. Pat. No. 3,413,144, which discloses a particular air-curable polyurethane for treatment or coating of textiles and paper.
Still, the particular systems of Igarashi et al and LaCombe et al are not suitable for use in the selective reinforcement of paperboard cartons and to our knowledge no one has previously suggested area-wise impregnation of strategic portions of paperboard cartons with a polyisocyanate.
As such, the need remains for an improved system for selectively reinforcing paperboard cartons to prevent mechanical failure of those cartons in use and, possibly, permit use of lower caliper paperboard which still has sufficient tensile strength and tenacity that such failures can be minimized.